The present application claims priority to Japanese Application No. P2000-144042 filed May 16, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a charging method for charging a nonaqueous electrolyte secondary battery (for instance, what is called, a lithium-ion secondary battery), and more particularly to a completely new charging method by which the high performance of the nonaqueous electrolyte secondary battery can be sufficiently achieved.
A battery has recently occupied an important position as the power source of a portable electronic device. In order to realize a compact and light device , it has been necessary for the battery to decrease its weight, and accordingly, such a battery as to efficiently use an accommodation space in the device has been demanded.
A lithium battery high in its energy density and output density is most suitable for meeting these demands.
The lithium secondary battery having the above described high performance has been further requested to have a higher performance so as to meet circumstances where the device with high performance is developed.
At that time, there arises a problem that an electrolyte is inconveniently decomposed on the surface of the anode of the battery.
Since the anode of the lithium secondary battery is exposed to a strong reducing atmosphere, the anode of the lithium battery in its charged state is naturally extremely apt to react to the electrolyte. Thus, the electrolyte employed in the lithium secondary battery forms a metastable state by producing a passive coat on the surface of the anode.
However, this coat undesirably causes the capacity of the battery to be decreased. At the time of initially charging after the battery is assembled, a certain degree of amount of charge carrier is employed to form the coat. Therefore, for instance, if the lithium doping/dedoping capability of a carbon material is examined in a test battery having a counter electrode composed of lithium metal, an initial discharge (lithium dedoping) capacity is smaller than an initial charge (lithium doping) capacity and the ratio thereof (initial charging and discharging efficiency=initial discharge capacity/initial charge capacity) is 80% to 95% in the case of a material which can be used as an anode material and the ratio is smaller in the case of carbon which cannot be used as an anode material.
In this case, the charge capacity is determined depending on the amount of an active material with which the battery is charged. The initial discharge capacity is equivalent to the capacity of the battery upon its use. Accordingly, a battery whose initial charging and discharging efficiency is low is disadvantageously equivalent to a wasteful battery whose amount of charge is large and a usable capacity is low.
The initial charging and discharging efficiency is dependent on a coat slightly formed on a cathode, the change of the crystal structure of the cathode as well as the coat forming reaction of the anode, however, the initial charging and discharging efficiency may be more greatly dependent on the anode. The passive coat on the surface of the anode is essentially required. Therefore, when this coat is generated with a minimum quantity of electricity, wastefulness can be reduced. Thus, the battery with high capacity and high energy density can be manufactured by charging the battery with the same battery material.
The present invention is proposed by considering the above described conventional problems and it is an object of the present invention to provide a new charging method in which the quantity of electricity necessary for forming a coat can be suppressed, an initial charging and discharging efficiency can be enhanced and the high performance (for instance, high capacity) of a nonaqueous electrolyte secondary battery can be sufficiently achieved.
The inventors of the present invention eagerly studied this object and found that a battery with high performance can be formed by generating a small quantity of stable coat on the surface of an anode. Since the quantity of electricity necessary for generating the coat is decreased, and accordingly, the charging and discharging efficiency can be enhanced, a discharge capacity can be increased. Further, a cyclic life characteristic is also improved without generating troubles for other battery performances.
The present invention was completed on the basis of the above described knowledge.
According to one aspect of the present invention, there is provided a charging method for initially charging a nonaqueous electrolyte secondary battery comprising an anode and a cathode and an electrolyte including a plurality of kinds of nonaqueous solvents, the nonaqueous solvents including a main solvent and one or more kinds of nonaqueous solvents having a reduction potential higher than that of the main solvent; the charging method comprising: charging steps of two or more stages including a first charging step in which the potential of the anode is maintained in a potential where any one of the nonaqueous solvents having the reduction potential higher than that of the main solvent is reduced and decomposed and the main solvent is not reduced nor decomposed to achieve a first charging process.
Further, according to another aspect of the present invention, there is provided a charging method for initially charging a nonaqueous electrolyte secondary battery comprising an anode and a cathode and an electrolyte including a plurality of kinds of nonaqueous solvents, the nonaqueous solvents including a main solvent and one or more kinds of nonaqueous solvents having a reduction potential higher than that of the main solvent; the charging method comprising: charging steps of two or more stages including a first charging step in which the value of the end voltage of a constant-current charge or the value of the constant-voltage of a constant-current and constant-voltage charge is set so that the potential of the anode ranges from 0.7 V to 3.0 V by using the potential of lithium metal as a reference to achieve a first charging process.
In the charging method according to the present invention, the first charging process is carried out by the charging step having two or more stages so that the initial charging and discharging efficiency is enhanced, the preparation of a wasteful material is decreased and high capacity and cyclic characteristics are improved without deteriorating various characteristics of the battery.
In order to realize the above operation, it is necessary to include a nonaqueous solvent decomposed under a potential higher than the reduction and decomposition potential of a main solvent in an electrolyte and this charging method serves to draw out the addition effect of such a nonaqueous solvent to its maximum.
As one specific means of the charging method, an electrolyte to which vinylene carbonate is added is used to initially carry out a constant-current and constant-voltage charge of about 3.2 V for 1 to 2 hours. Thus, a desirable coat can be formed on the surface of an anode.